This invention relates generally to improvements to "analog" wristwatches which are illuminated for telling the time in the dark. More particularly, the invention relates to an improved illuminated dial for an analog watch.
Electroluminescent devices were allegedly proposed by G. Destrau, London, Edinburgh, and Dublin Philosophical Magazine, Series 7, Volume 38, No. 285, Pgs. 700-737, October, 1947. There are a number of U.S. Pat. Nos. such as 2,988,661--Goodman and 2,928,974--Mash and 3,749,977--Sliker which describe the basic electroluminescent lamp. Such a lamp may comprise a sheet of glass or plastic with a conductive layer which acts as a first electrode, an electroluminescent layer comprising phosphor in a binder such as epoxy resin and a conductive sheet on the other side of the electroluminescent layer which serves as a second electrode. The resulting electroluminescent device is basically a capacitive circuit element, and when an alternating or pulsed voltage is applied across the two electrodes, the phosphor will illuminate or emit light in various colors depending upon the phosphor employed.
Electroluminescent devices have been proposed for two purposes in timepieces:
The first proposal is to use the electroluminescent device as a lamp to serve as a backlight for a transparent electro-optical display such as a liquid crystal display. The electroluminescent device does not provide any assistance to timekeeping other then as a source of illumination for the electro-optic display which indicates the time. Exemplary patents showing this use are seen in U.S. Pat. Nos. 4,208,869--Hanaoka; 4,238,793--Hochstrate; and 4,500,173--Leibowitz, et al.
The second proposal for utilizing electroluminescent devices in a timepiece is entirely different and suggests utilizing a group of radial segments or circumferentially spaced segments which are separately and selectively energizable to indicate the time. Exemplary patents utilizing selectively energized electroluminescent segments are U.S. Pat. Nos. 3,194,003--Polin; 3,258,906--Demby; 3,276,200--Freeman; and French Patent No. 1316428.
It is also known that a liquid crystal display with a central hole in it may serve in a dual capacity as the dial for a wristwatch with conventional hands as shown, for example, in U.S. Pat. No. 4,488,818. Also, it is known to utilize electroluminescent devices as backlighting for numerals printed directly on the electroluminescent device itself as shown in U.S. Pat. No. 4,532,395--Zukowski for flexible illuminated push buttons.
One of the difficulties in the past with utilizing electroluminescent devices in timepieces was the requirement of high voltage needed to produce sufficient light from the device, whereas modern electronic timepieces operate on low voltage using a single energy cell of only one and a half volts. This requires special circuits to boost the voltage for upgrading the supply voltage to the electroluminescent device and results in additional power consumption. However, it is known that losses can be reduced and still obtain acceptable brightness in relatively large panels of 0.1 square meter by connecting the capacitive panel with an inductor in a resonant circuit with the frequency of an AC source being adjusted to the resonant frequency of the circuit, according to U.S. Pat. No. 3,749,977--Sliker, issued July 31, 1973. In the Sliker patent, the substrate and the electroluminescent film are both disposed between the electrodes and selected to have equivalent electrical loss characteristics not to exceed a specified factor.
Accordingly, one object of the present invention is to provide an improved electroluminescent device for a conventional analog timepiece which enables reading the time at night.
Another object of the invention is to provide an improved electroluminescent device for use in a timepiece which operates at lower voltage.
Still another object of the invention is to provide an improved dial for reading an analog timepiece at night and a process for making it.